This invention arose in part out of research pursuant to Contract No. F336 15-99-C-2970.
This invention relates to controlling the flow of current to windings used in rotating machinery, and more particularly to controlling the flow of current to superconducting windings.
Superconducting windings are being used in electrical machinery and rotating machines because of their low loss characteristics. While the superconducting windings are maintained at cryogenic temperatures, the power supplies used to drive the superconducting windings are typically maintained at ambient temperatures (300xc2x0 K).
In the design of electrical machinery, incorporating high temperature superconducting (HTS) windings (i.e., motors, generators, magnets), the heat leak associated with the leads carrying current from the power supply at ambient temperatures to the cryogenically cooled windings is an overriding design factor which dictates the cost and thermal capacity of closed-cycle cryogenic cooling apparatus. These losses increase as the temperature difference between ambient and coil temperature increases. A number of approaches have been suggested to minimize the impact of heat leaks in such systems especially those in which the leads carry currents approaching 1 KA. Unfortunately, where vapor cooling of leads is not an option, these approaches introduce high voltages into the system or do not eliminate the need for a high current lead pair entering the cryogenic environment with attendant heat leaks. In cases where the superconducting coil is rotating with respect to a warm stator coil, the problem of heat leaks into the cryogenic environment becomes more critical due to the design constraints imposed by the thermal path impedance of a stationary cryocooler coupled indirectly to a rotating heat load or constraints on the size, weight, and thermal capacity of a rotating cryocooler.
There exist a number of large scale commercial and defense applications of HTS coils (e.g., magnet systems, generators and synchronous motor field windings) which require relatively constant magnetic fields, and in which ample time is available to ramp the coil current up to its initial desired value prior to regulated operation. In electrical machine systems incorporating HTS windings, the current in the HTS coil is subject to flux creep due to the finite losses in the HTS conductor. The dissipation due to this finite albeit small resistive loss requires that the current be restored periodically, i.e., xe2x80x9cpumpedxe2x80x9d via regulating circuitry back to its desired level. The energy input requirement is only that required to make up for the flux creep. Electronic circuits and mechanisms, which perform these functions, are referred to as xe2x80x9cflux pumpsxe2x80x9d.
The invention features an exciter assembly and approach for supplying power to a superconducting load, such as a superconducting field coil, disposed within a cryogenic region of a rotating machine. The exciter assembly provides an efficient and reliable approach for transferring the electrical power energy across a rotating interface and for controlling the ramp up and regulation of field excitation current in the field coil. In particular, the invention provides a controlled recirculation path for current flowing through the field coil.
In one aspect of the invention, the exciter assembly includes a transformer having a primary winding and a secondary winding, a sensor which provides a control signal indicative of the flow of field excitation current to the superconducting load; and a current regulator which is disposed in the rotating reference frame and, on the basis of the control signal, regulates the field excitation current to a predetermined value. The secondary winding is positioned in a rotational reference frame relative to the primary winding.
In essence, the current regulator provides a controlled recirculation path for current flowing through the super conducting load. By monitoring the flow of excitation of current in the load, once the desired level of current is provided in an initial charge up period, current to the load need only be provided relatively infrequently and for very short durations. The persistence characteristic of the coil current achieved in the power electronic control permits the exciter primary side source of AC signal to be turned off during the persistence phase. This reduces both core and winding losses and thus permits a considerably reduced winding rating in the exciter transformer. Moreover, by intelligently controlling the flow of current, the size, weight, and voltage rating of associated components for providing power (e.g., exciter transformer) can be significantly reduced, thereby increasing the overall efficiency and decreasing the cost of the system. This approach for supplying power to superconducting loads is particularly well suited for HTS superconducting rotating machines, such as those described in co-pending applications, Ser. No. 09/415,626, entitled xe2x80x9cSuperconducting Rotating Machinesxe2x80x9d, filed Oct. 12, 1999, and Provisional appl. No. 60/266,319 , entitled xe2x80x9cHTS Superconducting Rotating Machinexe2x80x9d, filed Jan. 11, 2000, both of which are incorporated by reference.
Embodiments of this aspect of the invention may include one or more of the following features.
The current regulator includes a first switching device in series between the secondary winding and the superconducting load, a second switching device in parallel with the superconducting load and between the first switching device and superconducting load, and a capacitor disposed in between the secondary winding and the first switching device and in parallel with the second switching device. The first switching device is closed when the second switching device is open to provide recharging current to the superconducting load, and the second switching device is closed when the first switching device is open to shunt current for recirculation through the superconducting load.
In one embodiment, the first and second switching devices are disposed within the cryogenic region, for example, the same region within which the superconducting load is disposed. In this case, the first and second switching devices are preferably metal oxide semiconductor devices. Cryogenic cooling of metal oxide semiconductor devices has been shown to decrease their on-resistance characteristics, thereby further reducing losses in the recirculation loop.
In an alternative embodiment, the current regulator is disposed in a non-cryogenic environment. Thus, cryogenic cooling is limited solely to the superconducting load. Such an arrangement allows the use of higher voltage semiconductor devices including an insulated gate bipolar transistor and a fast recovery rectifier for the first and second switching devices, respectively. Complexity of the assembly and associated drive electronics is reduced because large power blocks can be used instead of array of MOSFETs. Although more power is dissipated in the higher voltage, non-cryogenically cooled devices, the power is dissipated outside of the cryogenic environment and sufficient mass is available to cool the devices without complex thermal management. Moreover, in the event of failure of the switching devices or associated electronics, repair and maintenance is facilitated since there is no need to open the cryostat to gain access to the switching devices.
The load is a superconducting coil including high temperature superconductor. The primary winding is in the form of a stationary disk and the secondary winding is in the form of a rotatable disk axially spaced from the stationary disk to form a gap therebetween. In essence, the rotating disk and stationary disk provide a transformer for inducing AC voltage and current in the superconducting load. In one embodiment, the stationary disk and the rotatable disk are formed of radial laminations.
In all of the embodiments described above, the exciter assembly can further include a resistive load and a switch for allowing energy from the superconducting load to flow to the resistive load in the event of a detected fault.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.